The present invention generally relates to a keypad monitor and more particularly to a programmable keypad monitor which accommodates keypads having different numbers of keys. The present invention still more particularly relates to such a keypad monitor which is implemented in integrated circuit form and wherein, through its programmability, efficient use may be made of the integrated circuit pins when keypads of reduced size are to be monitored. The keypad monitor of the present invention disclosed herein is incorporated into a hands-free, multiple feature telephone unit adapted for use in an Integrated Services Digital Network (ISDN).
Keypads and monitors associated therewith are well known in the art. Such keypads generally include an array of switches with each switch coupled between a respective different pair of first and second conductors. When a keypad switch is depressed or closed, a condition can then be sensed across the first and second conductors to which a particular switch is coupled to identify which switch is being depressed. Such condition sensing is performed by the keypad monitor.
Keypad monitors for identifying a particular keypad switch being closed have been implemented in integrated circuit form. Such integrated circuits generally include a plurality of input terminal pins including a first group of terminal pins for connection to the first keypad conductors and a second group of terminal pins for connection to the second keypad conductors. The integrated circuits further generally include internal circuitry which scan the terminal pins for detecting the closure of a keypad switch.
The number of terminal input pins carried by these integrated circuits has been dictated by the number of keypad switches to be monitored. For example, if thirty-six keypad switches are to be monitored, these integrated circuits will include a first group of six input terminal pins and a second group of six input terminal pins. This arrangement is then capable of supporting the monitoring of thirty-six keypad switches arranged in a six-by-six matrix of keypad switches.
In the integrated circuit industry, the cost of producing an integrated circuit device is largely dependant upon the number of such integrated circuits to be produced. Hence, if one integrated circuit is able to function properly in a number of different applications, an economy of scale may be realized in producing the device. This not only lowers the per unit production cost of the device, but also lowers the per unit price for the device paid by the ultimate user.
The cost of an integrated circuit device is also dependant upon the number of terminal pins which must be incorporated with the device. As the number of terminal pins increases, the cost of the device correspondingly increases. Hence, in order to provide maximum functionality for a device, a device should preferably be designed to make efficient use of each terminal pin regardless of its application.
The present invention takes both of these important economic factors into account in providing an improved keypad monitor in integrated circuit form. First, the keypad monitor is programmable to support a full-sized keypad and a smaller-sized keypad. When programmed for monitoring the smaller-sized keypad, the terminal pins which are not utilized for connection to the first and second conductors are converted for other useful functions independent of the keypad switch monitoring function.